Resin-bonded abrasive articles, such as grinding wheels, typically are produced by mixing discrete abrasive particles with a liquid binder material and a powder resin and then pressing the mixture together under appropriate thermal conditions. The mixture can include other constituents such as, for example, fillers, curing agents, wetting agents and various metal powders.
A number of processes for fabricating abrasive articles employ novolac resins. In one approach, novolac resins are provided as solvent-based systems. One common disadvantage of this approach is ignitability of the solvent at high temperatures. Although solvent-free modified novolac resins have been developed, these materials are often expensive and their commercial use may not be economical.
Another approach for making abrasive articles is a multi-step process that includes applying a heated liquid novolac resin to the abrasive particles, followed by applying a dry bonding material. Typically, the dry bonding material includes another novolac resin and a curing agent. The sequence can be repeated until enough resin and bonding material has been added.
Some of the techniques currently available require the use of wetting agents, such as furfural. Wetting agents, in turn, can result in unstable molding mixtures and can generate a large amount of dust. Many known techniques require painstaking controls of temperature and/or addition of materials employed in order to obtain a suitably homogeneous granular molding material.
Therefore, a need exists for producing abrasive articles by methods which overcome or minimize the above-referenced problems.
The invention is directed to methods for producing a granular molding material for abrasive articles.
One method of the invention includes heating a granular abrasive material, blending a first phenol-novolac resin with a second phenol-novolac resin to form a resin blend and mixing in a vessel the heated granular abrasive material and the resin blend, thereby producing the granular molding material. Optionally, the vessel is preheated.
In one specific embodiment, the granular abrasive material is heated to a temperature in a range of between about 80xc2x0 C. and about 130xc2x0 C. In another embodiment, the temperature of the granular abrasive material is controlled by directing a gas across the vessel. In yet another embodiment, the vessel is preheated by directing a heated gas across the vessel. In a preferred embodiment, the first novolac resin is blended with the second novolac resin by spraying the first novolac resin onto the second novolac resin. In another preferred embodiment, the second novolac resin is combined with a curing agent prior to forming the resin blend. In a further preferred embodiment, the second novolac resin is combined with a filler prior to forming the resin blend. In yet another preferred embodiment, the method further includes the steps of molding and curing the granular molding material.
Granular molding material for abrasive articles can be prepared in an apparatus of the invention which includes a mixing vessel and means for directing a heated gas across or into the vessel. Optionally, the vessel is insulated with an insulation material.
The invention has numerous advantages. For example, the invention can be used to produce flowable granular molding materials which are homogeneously coated with phenol novolac resins. The materials obtained by practicing the invention give uniform tool structure with evenly distributed abrasive grain. The amount of loose, non-adhering material left in the mixing vessel after removal of the granular molding material is reduced or minimized. Compared to other methods, the invention also is advantageous with respect to dust generation and the stability of the granular molding material produced. The invention also reduces difficulties commonly associated with metering of resin. Further, the invention can be conducted employing simple, independent and flexible temperature controls. By using a preheated vessel in combination with preheated abrasive grain, a continuous rather than a batch-mode process can be employed to produce granular molding material.